Archivi tag: particles accelerators

Evidence for single top quark production through the weak nuclear force

Dopo una serie di tentativi che durano ormai da circa 20 anni, finalmente gli scienziati che lavorano agli esperimenti CDF e DZero presso il Fermi National Accelerator Laboratory hanno annunciato di aver trovato il modo di produrre un quark top. I due gruppi hanno affermato di aver osservato uno dei tanti metodi decisamente rari di produrre questa particella attraverso la forza nucleare debole, nel cosiddetto “canale-s”. Per arrivare a questo risultato, i ricercatori hanno dovuto analizzare più di 500 trilioni di collisioni protoni-antiprotoni che sono state realizzate con l’acceleratore Tevatron tra il 2001 e il 2011. I risultati indicano che in circa 40 collisioni, dove è stata prodotta la forza nucleare debole, sono stati identificati singolarmente quark top assieme a quark bottom.

Top quarks are the heaviest and among the most puzzling elementary particles. They weigh even more than the Higgs boson, as much as an atom of gold, and only two machines have ever produced them: Fermilab’s Tevatron and the Large Hadron Collider at CERN. There are several ways to produce them, as predicted by the theoretical framework known as the Standard Model, and the most common one was the first one discovered: a collision in which the strong nuclear force creates a pair consisting of a top quark and its antimatter cousin, the anti-top quark. Collisions that produce a single top quark through the weak nuclear force are rarer, and the process scientists on the Tevatron experiments have just announced is the most challenging of these to detect.

This method of producing single top quarks is among the rarest interactions allowed by the laws of physics.

The detection of this process was one of the ultimate goals of the Tevatron, which for 25 years was the most powerful particle collider in the world. “This is an important discovery that provides a valuable addition to the picture of the Standard Model Universe”, said James Siegrist, DOE associate director of science for high energy physics. “It completes a portrait of one of the fundamental particles of our universe by showing us one of the rarest ways to create them”. Searching for single top quarks is like looking for a needle in billions of haystacks. Only one in every 50 billion Tevatron collisions produced a single s-channel top quark, and the CDF and DZero collaborations only selected a small fraction of those to separate them from background, which is why the number of observed occurrences of this particular channel is so small. However, the statistical significance of the CDF and DZero data exceeds that required to claim a discovery. “Kudos to the CDF and DZero collaborations for their work in discovering this process”, said Saul Gonzalez, program director for the National Science Foundation. “Researchers from around the world, including dozens of universities in the United States, contributed to this important find”. The CDF and DZero experiments first observed particle collisions that created single top quarks through a different process of the weak nuclear force in 2009. This observation was later confirmed by scientists using the Large Hadron Collider. Scientists from 27 countries collaborated on the Tevatron CDF and DZero experiments and continue to study the reams of data produced during the collider’s run, using ever more sophisticated techniques and computing methods. “I’m pleased that the CDF and DZero collaborations have brought their study of the top quark full circle”, said Fermilab Director Nigel Lockyer. “The legacy of the Tevatron is indelible, and this discovery makes the breadth of that research even more remarkable”.

Fermilab: Scientists complete the top quark puzzle

Fermilab: Observation of s-channel production of single top quarks at the Tevatron
arXiv: Evidence for s-channel Single-Top-Quark Production in Events with one Charged Lepton and two Jets at CDF

arXiv: Search for s-channel Single Top Quark Production in the Missing Energy Plus Jets Sample using the Full CDF II Data Set

arXiv: Evidence for s-channel single top quark production in pp¯ collisions at s√ = 1.96 TeV
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Heavy quarks as ideal probes for quark-gluon plasma studies

Subito dopo il Big Bang, lo spazio era caratterizzato da una sorta di “zuppa primordiale” composta di quark e gluoni, ossia particelle di materia e di interazioni fondamentali. Questo plasma super denso si raffreddò quasi istantaneamente e la sua, seppure breve, esistenza contribuì sostanzialmente a creare le condizioni iniziali da cui si è successivamente evoluto il nostro Universo. Ma per capire meglio queste fasi iniziali della storia cosmica, gli scienziati devono ricreare nei grandi acceleratori di particelle quel plasma primordiale: è il caso del Relativistic Heavy Ion Collider (RHIC) presso il Brookhaven National Laboratory (BNL) dove si stanno analizzando i dati degli ultimi anni grazie ad un esperimento noto come STAR (Solenoidal Tracker at RHIC). A tale complesso è stato aggiunto di recente un nuovo rivelatore, denominato Heavy Flavor Tracker, il più avanzato nel suo genere e che servirà per studiare i processi di decadimento degli adroni costituiti da quark charm e bottom.

Scientists and engineers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab), have played a major role in the development of the STAR Heavy Flavor Tracker. The STAR HFT is actually the collective name for three separate silicon-based detector systems that make it possible for the first time to directly track the decay products of hadrons comprised of flavors (types) of quarks, “charm” and “bottom,” with heavy mass. Heavy quarks are considered ideal probes for quark-gluon plasma studies; however, their low production yield and short life-span (a fraction of a microsecond) make them difficult to study in heavy ion collisions that also produce huge quantities of light flavor particles. The HFT was first conceived nearly 15 years ago by Berkeley Lab’s Howard Wieman, a physicist with the Lab’s Nuclear Sciences Division who also played a prominent role in the creation of STAR. The HFT construction project, which began a few years later, was initially led at Berkeley Lab by Hans Georg Ritter, a physicist who served as head of the Nuclear Science Division’s Relativistic Nuclear Collisions program (RNC) for many years. “The HFT enables precision tracking measurements of heavy quarks at low momentum where the particle production is most sensitive to the bulk medium created in heavy ion collisions”, says Nu Xu, a physicist also with Berkeley Lab’s Nuclear Science Division who is the current spokesperson for the STAR experiment. “This allows us to distinguish the decay vertices of heavy flavor particles from primary vertices and significantly reduces combinational background, which yields cleaner measurements with a higher level of significance”. The importance of the HFT’s precision measurements at low momentum to quark-gluon plasma studies is explained by Peter Jacobs, a Berkeley Lab physicist who now heads the Nuclear Science Division’s RNC program. “Theorists claim they can calculate the dynamical behavior of heavy quarks in matter more accurately than that of light quarks or gluons. Some even think they can calculate the dynamical behavior of heavy quarks in the quark-gluon plasma using models inspired by string theory”, Jacobs says. “One of the things we will be testing with the HFT is the different predictions of the behavior of heavy flavors in the quark-gluon plasma made by string-inspired models versus more conventional physics”.

Berkeley Lab scientists and engineers are now developing a new, larger version of the HFT which they propose to be fabricated for the ALICE detector at CERN’s Large Hadron Collider. “If approved, this will be an upgrade to the Inner Tracking System of the ALICE experiment at the LHC that is a direct follow-on to the STAR HFT, utilizing a number of HFT developments”, says Jacobs. “It is proposed to be installed during the next long LHC shutdown in 2018 and will essentially be a 25 giga-pixel camera made up of 11 square meters of silicon, about 30 times larger than the HFT at STAR”.

LBL: Heavy Flavor Tracker for STAR

arXiv: Heavy Flavor Tracker (HFT) : A new inner tracking device at STAR